(WASHINGTON) — Most attacks on federal government buildings and officials were carried out by so-called lone wolves using firearms who exhibited “concerning” behavior beforehand, according to a new study by the U.S. Secret Service National Threat Assessment Center.

The Secret Service says many of the cases illustrate the need to better train law enforcement and civilians to recognize signs of these problems to increase the likelihood of preventing attacks in the future.

The center analyzed 43 incidents that occurred between 2001 and 2013 and found that all but one of the offenders exhibited “concerning” behavior before the attack.

“These behaviors went back anywhere from just a few days to several decades, in different venues the severity varied,” said Michelle Keeney, chief of the Secret Service National Threat Assessment Center.

Paranoia, depressive symptoms and delusions were the most common symptoms of mental illness in the population of study, according to Keeney.

“People with mental illness are no more likely to engage in violence than people without mental illness. But it is an important aspect of individuals who come to our attention. We look at it as a part of every investigation,” she said.

Difficulty coping with stress was also prevalent among the assailants. Over three-quarters of the offenders experienced a stressful event in the year prior to their attack.

In 2010, John Patrick Bedell calmly approached a Pentagon security check point and began firing without saying a word on the Pentagon Force Protection Agency officers. They returned fire and fatally wounded Bedell. Two officers were injured in the incident, captured on video exclusively obtained by ABC News.

Although the motive for this attack was unclear, Bedell had a long history mental illness. He believed the government was spying on him and believed in several anti-government and anti-military conspiracy theories, according to the report.

Keeney said early intervention is the key to stopping such attacks before they occur.

“We really want to work with the public and encourage them to develop interventions,” she said.

While most assailants used guns, others used poison and explosives and homemade devices.

Incendiary devices were mailed to top government officials, including former Maryland governor and presidential candidate Martin O’Malley and former Homeland Security Secretary Janet Napolitano.

Almost half of the offenders exhibited evidence of having a fixation with an individual, activity or idea, and over half had previously acted violent toward others. And while one-third had made threats or veiled references to harming their targets through such means as online postings or verbal comments, only three of the attackers communicated direct threats to their targets.

In 2013, Aaron Alexis entered the Navy Yard complex in Washington, D.C., opening fire on employees and civilians. He killed 12 people and injured three others before being shot to death by responding officers. In the weeks leading up to the attack, Alexis suffered from paranoia, hallucinations and delusions.

“I would encourage people again, if it’s safe, if they can intervene in some way,” Keeney said. “At these early stages at someone’s life or when they are first starting to have problems, that’s when we want to intervene.”